1. Field of the Invention
The present invention relates to hydraulic control apparatuses for automatic transmissions mounted in, for example, vehicles and, in particular, to a hydraulic control apparatus for an automatic transmission having a low speed achieved by engagement of a one-way clutch, forming a neutral condition by discharging a hydraulic pressure when a shift lever operation is performed to change from a forward range of a lower medium speed condition to another shift range.
2. Description of the Related Art
A multi-stage automatic transmission mounted in, for example, vehicles generally forms a shift speed by controlling a rotating condition of each of rotating elements in a speed change gear mechanism that combines together the rotating elements including, for example, a planetary gear. The multi-stage automatic transmission controls the rotating condition of the rotating elements through selective hydraulic engagement of a plurality of clutches or brakes, thereby enabling control of multiple shift speeds.
Among such automatic transmissions, some achieve a forward first speed by engaging an input clutch (C-1) with a one-way clutch (F-1), in addition to engagement of the clutches and brakes (see, for example, Japanese Patent Publication Application No. JP-A-2007-177934 which is hereinafter referred to as Patent Document 1). The one-way clutch performs a smooth automatic engagement to lessen an engagement shock particularly when the forward first speed during a change from a neutral range to a forward range is achieved.
A hydraulic control apparatus used in the automatic transmission includes a solenoid valve regulating and controlling an engagement pressure supplied to a hydraulic servo of the clutches and brakes and a manual shift valve allowing a spool position to be changed based on the shift lever operation. Specifically, the manual shift valve allows supply and discharge of a source pressure (a forward range pressure or a reverse range pressure) of the engagement pressure supplied to the hydraulic servo of the clutches and brakes to be collectively controlled. This reliably forms the neutral condition even if, for example, the solenoid valve fails, thus enhancing reliability and safety as the automatic transmission.
There is, however, the following problem. Specifically, a clutch C-1 and a brake B-1 are engaged as in, for example, the forward second speed of Patent Document 1 when an input shaft of the speed change gear mechanism is in a condition of being an idle speed or less as when, for example, the vehicle is stationary or is about to stop. In addition, the manual shift valve is changed from a forward range position to a neutral range position through the shift lever operation, forming a neutral condition by discharging a hydraulic pressure. If the engagement pressure of the brake B-1 is discharged before the engagement pressure of the clutch C-1 under the foregoing condition, the input shaft of the speed change gear mechanism is driven at a higher speed through the idle speed of the engine via, for example, a torque converter. As a result, the one-way clutch is automatically engaged to form, together with the clutch C-1, a forward first speed. Specifically, the forward second speed is changed to the neutral condition via the forward first speed, so that the neutral condition is established after a driving force is transmitted to a drive wheel with amplified torque of a gear ratio being instantaneously the forward first speed. This produces a shift shock not expected by a driver, posing a problem of the driver's not having a good shift feeling.